LED Color vs Performance -- A2, etc.

Hi Guys,

Coming down off of my latest flashlight binge, I have become interested in the optical performance of various colors of LEDs, particularly with the Surefire A2 flashlight.

The two that I have are red and yellow-green (YG). The red is for applications where I want to preserve my dark-adapted vision; the YG is for more general use, but where I want a soft dim light that is maybe better than white for not attracting insects.

A peculiarity that I have noticed is with the "range" of red. My red A2 is obviously brighter than the YG up close, but at any kind of distance (10 ft or more), the YG easily wins the brightness contest. Why is that? Do our eyes lose sensitivity to red quickly with reduction in intensity? Is it a beam quality thing? It is really strange to make the comparison close and far.

I also did the "map test" with some of my Surefire lights: L2, A2-RD, A2-YG. The L2 was the control, below.



Next was the A2-YG. The picture doesn't quite do it justice, since you can actually make out some colors by its light. Things are mostly monochrome, but in person you can make out some red, green, orange, etc. The only color that really washes out is yellow, not a big deal as few things are printed yellow on white. I don't know its dominant wavelength. This is a very useful color.



Last is good old red. Great for stargazing, etc, but lousy for maps. A note about photographing red: you have to refocus, as I suspect the focus point for red light is different than for more "normal" wavelengths -- same reason colors of white light split in a prism. I still missed the focus here, but you get the idea.




The YG really is easy to read by. I think it was the sliderule manufacturer Picket that did extensive testing with eye strain. They decided that the best color to use was a shade of yellow. Below is a pic of the model of slide rule that was sent on the Apollo missions to the Moon.



So, these are just food for thought about LEDs and color and vision. There are times, at least for me, when white light is either less functional or less fun /ubbthreads/images/graemlins/smile.gif

What are your thoughts?

Scott

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My red A2 is obviously brighter than the YG up close, but at any kind of distance (10 ft or more), the YG easily wins the brightness contest.

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Perhaps there is nothing more than a difference in beam divergence between the two? Is the overall area of illumination the same between these two?

The YG model looks great! I need to try one of these out. /ubbthreads/images/graemlins/cool.gif

It makes perfect sense that two colors will be better than one with a multicolor target. Try a B&W photo and it will be less noticable but an edge will go to the grn/yellow due to the eyes sensitivity to that spectrum.

Last is good old red. Great for stargazing, etc, but lousy for maps. A note about photographing red: you have to refocus, as I suspect the focus point for red light is different than for more "normal" wavelengths -- same reason colors of white light split in a prism. I still missed the focus here, but you get the idea.

Click to expand...

It is probably more of a consequence of the focusing mechanism not locking on. Most consumer cameras will use contrast to determine focus and with 2 times as many sensors in the ccd for green you will probably see the green/yellow being more effective. But before I eat crow, you are using autofocus and not manual focus right?

-CAL

Luna,

I am using a Canon Digital Rebel with an EF 50mm "ultrasonic" lens set on manual focus. The way I had the map and tripod set up required that I take a big image and crop to a small portion. I tried manually focusing, and using auto to set the focus, then switch to manual for the exposure.

I have tried this with other, cheaper lenses -- focusing in white and then leaving it. The red is even more out of focus through the cheaper lenses, making me think it is a lens design/refraction issue. I think the bottom line is that I am not particularly skilled at focusing on dim things in the dark /ubbthreads/images/graemlins/smile.gif

I'll see if I can get a sharper red pic tonight.

I am really curious what is going on with the YG and resolving colors. I don't know the spectrum from the LEDs, but I assume they emit a "single" color like other non-whites do. Maybe YG is centered enough in the visible spectrum that other colors are thrown out to either side of the peak wavelength. I dunno.

Scott

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beezaur said:
Luna,
I am using a Canon Digital Rebel with an EF 50mm "ultrasonic" lens set on manual focus.


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The lens must be treating the red light like IR then. They sometimes include IR focus/distance tick on the barrel because of it being a different focal point. It is odd it is happening with visible red. Could be the IR focus processor or the DIGIC proc thinking it is a an IR mode and compensating for the focal length difference?

Seems that you would never have a sharp picture with a subject that has red in it.

Well, time for you to get some "L" series glass just in case /ubbthreads/images/graemlins/smile.gif

-Craig

Those maps in the photos are ordinary road maps, aeronautical charts don't wash anywhere near that bad under red. The yellow-green A2 looks like it may be ideal for the road map and probably most terrain maps.

The level of washout experienced with charts used for instrument flying is virtually nonexistent because they were designed to be used in red light, and some are actually printed in plain black & white.

Richard.

I took some outside pictures last night, to try and shed some light /ubbthreads/images/graemlins/wink.gif on the percieved brightness of my A2's yellow-green (YG) over red at a distance. My eyes see the A2-RD as much brighter up close, like reading, etc. However, the A2-YG seems way brighter outside at a distance. My camera strongly disagrees, putting red much brighter than YG. Below are snaps (well, about 8 seconds worth of snaaaaaap) of each color, taken with identical settings in manual mode.





But like I said the red does not appear like that to me. The red I see looks more like this:



Really dim.

I know that some of the problem is the target -- green plants look green under yellow-green light, and black under red light. But I think the rest of the problem is the camera sensor. Tthe pics I took of supposedly black plants under red light turned out generally bright red (check the dandelion near center). Somehow the camera is detecting light that I cannot see.

There are really two sensors that I know very little about: my camera's sensor, and my retinas.

The best way I can describe the "red light at a distance" problem is to compare it to suddenly walking into a totally silent room. Your ears seem to strain to hear something, an almost uncomfortable reaching for input. The red light can make me feel that way, like my eyes are straining to detect light that is almost there. The YG is very comfortable by comparison, maybe like hearing pleasant music at low volume. Both colors are fine up close.

Scott

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It looks from the image that the red has a more even distribution of light and a larger area of coverage? Up close, the yg may have a smaller hot spot that you are not really noticing but at a distance, this concentration of light may be much more obvious./ubbthreads/images/graemlins/icon3.gif

The CCDs are extremely sensitive to red because of the IR focusing that is used. The are sensitive to IR (just look at a remote controls aimed at the camera while depressing a button) and it will appear white. A filter is placed in front of most ccds to filter out IR so as to not affect the picture but some cameras have defeatable ir filters (like the older sony xrayvision camcorder) that pick it up well.

So all in all, it makes sense that it picks up 650nm leds well.

Actually, the red is very "ringy," like the biohazard symbol, and has a significant hotspot. The YG is more diffuse, relatively even.

I believe it is a sensitivity issue of some sort. Maybe I have some sort of colorblindness that is threshold-sensitive (a wild guess). Some of the pictures I took were of grass where I could hardly tell the red light was on, but it showed up very bright in the pictures. The YG on the same scene came out more or less as I percieved it.

Another note about the red fucus issue: The plant scene above goes from out of focus in the background through a focused plane to out of focus in the foreground. Theoretically if there are different focal planes for different colors, it would be detectable in that pair of images. The full-size images just show crappy focus all the way through. So, at least with my expensive lens, there is not a significant difference in focal point.

Most likely the poor focus under red is due to the fact that red-sensitive receptors are few and far between on the camera's sensor. Compare that to YG light, which will be picked up by a lot more receptors, creating a much higher resolution image. But that is about the extent of my knowledge of such things, so I could well be wrong.

Scott

PS:

OK, Luna, that makes good sense to me about the camera being IR-sensitive. What would you suggest to get a more human-representative picture? Is there a filter that I can buy?

Scott

Scott,

You're not color blind, the red light will cause some people to have difficulty seeing things at intermediate distances. I know a lot of guys who after using red light can see well within about 3 feet, then not much of anything, but able to still see things at a distance past 20 or 30 feet. They're able to see at the intermediate distances again within a minute or so. For some reason that temporary focusing problem seems to affect me less than others. Red is also virtually useless until your eyes have adapted to the dark which takes at least 30 minutes, although partial adaptation can be gained while using red light. If using it after trying other colors of light, your dark adaptation is ruined and you'll see much less with the red. The yellow-green and greens and blues will ruin night vision the same as white.

Also since you're one of the first with a yellow-green A2, I'm wondering if you have had the chance to test that color around large numbers of bugs at night.

Richard.

Thanks Richard.

I have come to appreciate vision and color as a very strange and wonderful thing. You seem to be describing the same thing that happens to me. I wonder where a guy would find more information like that?

The yellow-green does not mess up my night vision too badly. By that I mean I acquire a certain amount of dark-o-vision after a few minutes (maybe when my pupils fully dilate?) which is persistent through use with the A2-YG. At least it is better than my L1 on low. I have not yet tested it for true dark-adapted vision, but have no doubts that it will have a negative effect.

I haven't had any bugs to speak of, but I am anxious to see how that goes. Soon.

Scott

Scott,

Most books that I've read that mention it really don't mention much beyond what is posted in this thread and tend to skip the information on the temporary vision loss at intermediate ranges, but if you're in a major city with well stocked bookstores, check for any books on night flying in the aviation section.

Also one of the more thorough works available that covers the aspects of both day and night vision can be read online here:

http://wwwsam.brooks.af.mil/af/files/fsguide/HTML/00_Index.html
USAF Flight Surgeon's Guide: Contents

The link to the USAF manual is courtesy of an older post by tylerdurden.

Richard.

Wow! That is a great link. I am not near any large cities, but I am near amazon.com /ubbthreads/images/graemlins/wink.gif I'll have to do some looking.

Scott

Scott,

If amazon.com is anywhere near snake country it would be nice to know if it's easy to see snakes and scorpions and other things that are best avoided while using the yellow-green A2...not wanting to sound cowardly but like Daffy Duck said, "I'm allergic to pain" and I spend several months a year in snake infested locations.

Richard.

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beezaur said:
Wow! That is a great link. I am not near any large cities, but I am near amazon.com /ubbthreads/images/graemlins/wink.gif I'll have to do some looking.

Scott

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Check this out http://stlplaces.com/night_vision.html .

I have no problem with human eyes ability to focus oddly at night. You have to look about ~10deg away from the subject at night for the best picture all because of the distribution of rods and cones.